Conductive inks have been used for many years in a variety of applications. For instance, conductive inks have been printed on various substrate materials to form conductive circuits as are found in radio frequency identification tags, touch pads, and flexible circuits. Traditionally, conductive inks used for such applications included a dispersion of conductive particles, either metal (e.g., silver) or carbon, in a resin system designed to meet the specifications of both the formation process and the end-use application. Conductive inks have greatly reduced both the monetary and environmental costs associated with various types of products such as circuits, as the traditional etching process for forming electronic circuits required many separate steps and generated significant hazardous waste problems. U.S. Pat. No. 5,189,952 to Ohmura, et al., U.S. Pat. No. 4,221,830 to Dalens, et al., U.S. Pat. No. 5,286,415 to Buckley, et al., U.S. Pat. No. 5,389,403 to Buckley, et al., and U.S. Pat. No. 5,656,081 to Isen, et al. exemplify several metal and/or carbon particle-containing conductive inks.
Recently, intrinsically conductive polymers (ICP) have been examined as possible replacement materials for the conductive particles found in more traditional conductive inks. For instance, ICP inks have been suggested for use in forming printable anti-static coatings, smart windows, corrosion control layers, EMI/RFI shielding, and in photovoltaic applications. U.S. Pat. No. 6,358,437 to Jonas, et al., U.S. Patent Application Publication 2005/0070654 to Hsu, and European Patent Application Publications EP-A 1 081 549 and EP-A 1 081 548, exemplify some recent trends toward forming printable or otherwise castable compositions through inclusion of conductive polymers in the formulations. For instance, Hsu discloses a composition including an aqueous dispersion of electrically conducting organic polymers and a plurality of nanoparticles. The films cast from the composition are useful as buffer layers and have a much lower conductivity than a similar buffer layer without the nanoparticles.
One of the more recent advances in the utilization of ICPs has been in the development of electrochromic devices (ECD) that include electroactive materials capable of generating a visible, chromatic effect upon change in electrical state of the materials. For instance, U.S. Pat. No. 5,500,759 to Coleman describes an ECD including an essentially ionically isolative composite layer including a dispersion of electrically conductive polymers in a polymer matrix. U.S. Pat. No. 6,501,587 to Ferraris, et al. relates to an apparatus, a process, and a resulting device that can be used to create an image using an electrochromic polymer ink. The electrochromic polymer ink of Ferraris, et al. includes a colloidal electrochromic polymer pigment dissolved in an aqueous solvent solution.
Problems and room for improvement still exist in the art. For instance, conductive polymer inks that could be used in high-speed, non-contact printing processes such as inkjet printing would be beneficial as such printing processes can generate high resolution patterns at high speed using a small volume of materials. It has proven difficult, however, to provide electrochromic inks capable of forming on a substrate a pattern of uniform coating that can exhibit high resolution, good conductivity and robust adhesion.
What are needed in the art are conductive polymer inks that can be formulated with characteristics suitable for desired printing processes, and in one particular embodiment, for inkjet printing processes.